1. Field of the Invention
The present invention relates to an exposure condition determining method and a photographic printing apparatus to which the exposure condition determining method is applied.
2. Description of the Related Art
Light of three colors, B (blue), G (green) and R (red), is transmitted through the entire image surface of an image recorded on a color negative film. It is known that, in general, the ratio of the three colors transmitted through the image is substantially equal or the ratio is fixed. Therefore, the exposure condition is determined in a photographic printing apparatus in accordance with the following formula: EQU log Ej=Kj+Dj (1)
wherein log E is the logarithm of the amount of exposure E, K is a constant, D is the light accumulated transmission density (LATD) of an image photometrically measured by a photometric system, and j is any of R, G, and. B light. When an image is printed at the photographic printing apparatus at an exposure condition determined on the basis of above Formula (1), the overall density of a print produced from an underexposed image is high compared with the density of a print obtained from an appropriately exposed image, whereas the density of a print produced from an overexposed image is low. Therefore, the value Dj in Formula (1) is corrected by effecting slope control, and the exposure condition is determined. However, characteristics of films, such as sensitivities and densities of the three photosensitive layers, are different for different types of films which are categorized in accordance with the manufacturer and the sensitivity thereof. When different types of films are printed/exposed under the same exposure condition determined as described above, constant and high-quality prints cannot be obtained.
As a result, as a method of correcting for the differences between the types of films and effecting appropriate printing, a method and apparatus have been proposed for prescribing the amount of copy light when a color original is copied (Japanese Patent Application Laid-Open No. 55-46741). In this method of prescribing the amount of light, a plurality of image frames recorded on one roll of a film, which is to be printed, is photometrically measured. A characteristic curve of the difference in color densities related to the average density is prepared, and a film characteristic value expressing the characteristic of the film is determined. Further, the photometric value (the original characteristic value) of an image frame to be printed is corrected by the film characteristic value so that the value Dj in Formula (1) is obtained and the exposure condition is determined.
The photometric value of the image frame recorded on the film is effected by the characteristics and the like of the film. The film characteristic value obtained by the above-described method reflects the film characteristic of an individual film. Accordingly, an exposure condition corrected for differences in the film types is obtained by using the film characteristic value. Further, when the film has deteriorated over time and the colors of the image frames have faded, it is preferable to effect printing at an amount of exposure which is varied from a standard amount of exposure in accordance with the extent of deterioration caused by time. In the aforementioned method of prescribing the amount of light, the photometric value varies in accordance with the variation in the film characteristic due to deterioration over time. The photometric value varies in accordance with the variation in the film characteristic value. Therefore, high-quality prints in which the effects of deterioration due to time have been corrected are obtained. Moreover, good results in correcting the effects of deterioration due to time are also obtained in cases in which most of the image frames recorded on the film have been photographed by heterogeneous light sources other than sunlight.
However, there are cases in which a large number of the image frames recorded on a single roll of film are prone to color failure. In such cases, there is a drawback in the above method of prescribing the amount of light in so far as good results cannot be obtained. Examples of image frames prone to color failure include image frames in which the photographed subject is green, such as trees, flowers and the like, or image frames including a blue summer sky and the sea. Further, image frames photographed on lawns in parks or the like and sport scenes in which subjects are photographed under bright sunlight are also susceptible to color failure. When a large number of these image frames prone to color failure are included successively on a single roll of film, it is difficult to correctly determine the film characteristic value.
Reasons for this difficulty are that in image frames prone to color failure, as the density of a specific color is high and the color balance is deviated from that of an image frame of a general, photographed subject, the film characteristic value of a film on which many images susceptible to color failure are recorded varies due to the effects of the color failure and does not adequately reflect the film characteristic. Correct determination of the film characteristic value is therefore difficult. When the image frames are printed at an exposure condition determined by using the film characteristic value of a film having many images prone to color failure, the main subject is printed at a color balance which deviates from the correct color balance. This effect occurs even when, for example, about half of the number of the image frames of a single roll of film are susceptible to color failure. Therefore, this problem occurs quite frequently.
An apparatus for determining an amount of exposure for photographic printing is proposed in Japanese Patent Application Laid-Open No. 2-93448 as another means for correcting for the differences between film types, and effecting appropriate printing. This apparatus includes a storing means which reads a code of the film type recorded on films, classifies the films per film type based on the code, and stores the image density for each film type. In this apparatus, image densities are calculated by using photometric values belonging to a specific color region based on the stored image densities, so as to determine the amount of exposure. Further, a method of determining amount of exposure for an image copying apparatus has been proposed in Japanese Patent Application Laid-Open No. 2-90140. In this method, a determination of the amount of exposure is made by detecting where photometric data regarding the three colors of the film belong to any of color regions on a color coordinate system, which is divided into a plurality of these color regions, and the photometric data are classified. Then, the amount of exposure is determined based on a first image data belonging to a color region in which the color difference or the color ratio from a reference value is low, and a representative value of the first image data, or the reference value.
The above systems are based on the premise that the films of the same type have similar characteristics, and the color balance and the like of image frames recorded thereon are similar. Over a long period of time, data obtained from a large number of films is stored for each type of film, and the exposure condition is determined based on the data for each type of film and on the data for the image frame to be printed. Accordingly, even in cases in which films which have many images prone to color failure recorded thereon are to be printed, the effect due to the color failure can be almost completely eliminated. However, compared with the above-described method of prescribing the amount of light, even if, for example, the film has deteriorated due to the passage of time, the variation in the film characteristic due to this deterioration is not easily reflected in the exposure condition, and there is a problem in that high-quality printing results cannot be obtained. This problem also occurs when image frames which were photographed under heterogeneous light sources are to be printed.
In recent years, a number of high-sensitivity films has been developed. Currently, there is a very large number (several tens) of types of high-sensitivity films. A film is classified into one of the many types of films based on the manufacturer of the film or the sensitivity thereof. Films of different types differ in their film characteristics, which include the sensitivities of three photosensitive layers, the density of a base layer, and the like. Therefore, the optimal exposure conditions are not always the same.
Further, images of a standard subject, which is photographed at correct exposure, underexposure and overexposure, are recorded on exposure condition setting films which are used when the exposure condition of the image frame recorded on the film is set. However, these exposure condition setting films are only prepared for an extremely limited number of film types. The setting of exposure conditions of film types for which an exposure condition setting film has not been prepared is carried out by trial and error. Therefore, time and experience are necessary to set the exposure condition.
From this point of view, the following technologies have been developed and explained in Japanese Patent Application Laid-Open No. 63-311241 and the previously-discussed Japanese Patent Application Laid-Open No. 2-93448. A reading means for reading a DX code, which is recorded in advance on the film and expresses the film characteristic and the like, is provided. The film type is determined based on the read DX code. The image data is stored in a memory corresponding to the film type. The exposure condition is determined by using the image data corresponding to the film type of the film which is to be printed. Accordingly, the exposure condition is determined based on the stored image data for each type of film and on a reference exposure condition. Therefore, even if an exposure condition setting film, which corresponds to the film type of the film to be printed, is not available, an accurate exposure condition can easily be obtained.
However, currently, new films are being developed every year, and the manufacturing of older films is often stopped. In the above-described technologies, the storage regions for storing the data for determining the exposure conditions are not managed. Therefore, a drawback arises in that the limited storage regions, which are provided with a memory or the like, are occupied by data regarding types of films which are no longer manufactured or data regarding types of films which were developed long ago and are used infrequently.
In order to solve this drawback, a film managing apparatus has been proposed in Japanese Patent Application Laid-Open No. 2-93449. In this apparatus, a determination is made whenever necessary as to whether there are film types, among the film types stored in the memory of the apparatus, having similar film characteristics. When there are similar film types, the respective data thereof are integrated, and one of the data is deleted. In this way, necessary storage regions can be conserved. However, there are drawbacks in that it is not easy to automatically determine with accuracy whether the film characteristics are similar and in that the precision of the data deteriorates.